The 91 departs NYP at about 11AM so probably transits the ERT at about 10. I suspect this is during a mid-morning lull after the morning commute. The 92 is scheduled to arrive at NYP at about 7PM, but often (both today and tomorrow's arrival is early, by as much as an hour according to the transitdocs map, so would transit the ERT between 6:30 and 7:30, at the end of the evening rush. They are also occupying a platform at NYP for a significant amount of time, thus blocking other trains from transiting the tunnels. I don't think this is very much, nor at the worst possible times, but I suppose every bit helps. I suspect the equipment utilization issue has much greater impact, but maybe Amtrak decided it was harder to explain. It also begs the question why (if this true), they didn't include the CL in the new bi-level RFP (or did they?)I get fleet utilization efficiencies being a tangible reason for this change, but does this really make that big of an impact with regard to the East River Tunnel Project? Is it worth the incredible work required to make this change only temporary?
I think making the Floridian permanent and bi-level (after the bi-level equipment is delivered) would make the most sense. I assume delivery of the new bi-levels would take a couple of years, but converting the Floridian to bi-level could be done using retiring Superliners as soon as enough "retired" Superliners become available.
Keeping the Floridian and re-instating the single-level Star on the current schedule would be redundant south of DC, spreading out the arrival and departure times is in principle better. If they want to expand Florida service, I think re-instating the Star on a modified schedule or extending the Palmetto (which IIRC was originally the NYP to Florida Silver Palm), if the schedule works better, should be pretty easy, once the ETR work is done.
As to your second question, "Is it worth the incredible work required to make this change only temporary?", as things go, I think it was not "incredible work" to replace the CL and SS with the Floridian. No new track, no new stations or platforms required, existing schedule slots, saving an engine change in DC. This is just about the simplest "new" long distance service imaginable.
(Note I have no particular expertise. All of my railroading experience is based on being a passenger and playing with Lionel and HO trains as a child.)